+ linkend="sql-syntax-type-casts">. But the form
+ type 'string'
+ can only be used to specify the type of a literal constant.
+ Another restriction on
+ type 'string'
+ is that it does not work for array types; use ::
+ or CAST() to specify the type of an array constant.
- A column reference
+ A column reference.
- An operator invocation:
-
- expression operator expression (binary infix operator)
- operator expression (unary prefix operator)
- expression operator (unary postfix operator)
-
- where operator follows the syntax
- rules of or is one of the
- tokens AND, OR, and
- NOT. Which particular operators exist and whether
- they are unary or binary depends on what operators have been
- defined by the system or the user.
- describes the built-in operators.
+ A positional parameter reference, in the body of a function declaration.
-( expression )
- Parentheses are used to group subexpressions and override precedence.
+ An operator invocation.
- A positional parameter reference, in the body of a function declaration.
+ A function call.
- A function call
+ An aggregate expression.
- An aggregate expression
+ A type cast.
- A scalar subquery. This is an ordinary
- SELECT in parentheses that returns exactly one
- row with one column. It is an error to use a subquery that
- returns more than one row or more than one column in the context
- of a value expression. (But if, during a particular execution, the
- subquery returns no rows, the scalar result is taken to be NULL.)
- The subquery can refer to variables from the surrounding query,
- which will act as constants during any one evaluation of the subquery.
- See also .
+ A scalar subquery.
+
+
+
+
+( expression )
+ Parentheses are used to group subexpressions and override precedence.
the key words NEW or OLD.
(NEW and OLD can only appear in the action portion of a rule,
while other correlation names can be used in any SQL statement.)
- The correlation name can be omitted if the column name is unique
+ The correlation name and separating dot may be omitted if the column name
+ is unique
across all the tables being used in the current query. If
column is of an array type, then the
optional subscript selects a specific
- element in the array. If no subscript is provided, then the whole
- array is selected. Refer to the description of the particular
- commands in the PostgreSQL Reference Manual
- for the allowed syntax in each case.
+ element or elements in the array. If no subscript is provided, then the
+ whole array is selected. (See for more about
+ arrays.)
+
+
Operator Invocations
+
+ There are three possible syntaxes for an operator invocation:
+
+ expression operator expression (binary infix operator)
+ operator expression (unary prefix operator)
+ expression operator (unary postfix operator)
+
+ where the operator token follows the syntax
+ rules of or is one of the
+ tokens AND, OR, and
+ NOT. Which particular operators exist and whether
+ they are unary or binary depends on what operators have been
+ defined by the system or the user.
+ describes the built-in operators.
+
+
+
Function Calls
where aggregate_name is a previously
defined aggregate, and expression is
- any expression that does not itself contain an aggregate
+ any value expression that does not itself contain an aggregate
expression.
+
+
Type Casts
+
+
+ type casts
+
+
+ A type cast specifies a conversion from one datatype to another.
+
PostgreSQL accepts two equivalent syntaxes
+ for type casts:
+
+CAST ( expression AS type )
+expression::type
+
+ The CAST syntax conforms to SQL92; the syntax with
+
:: is historical
PostgreSQL+ usage.
+
+
+ When a cast is applied to a value expression of a known type, it
+ represents a run-time type conversion. The cast will succeed only
+ if a suitable type conversion function is available. Notice that this
+ is subtly different from the use of casts with constants, as shown in
+ . A cast applied to an
+ unadorned string literal represents the initial assignment of a type
+ to a literal constant value, and so it will succeed for any type
+ (if the string literal's contents are acceptable input syntax for the
+ datatype).
+
+
+ An explicit type cast may be omitted if there is no ambiguity as to the
+ type that a value expression must produce (for example, when it is
+ assigned to a table column); the system will automatically apply a
+ type cast in such cases.
+
+
+ It is also possible to specify a type cast using a function-like
+ syntax:
+
+typename ( expression )
+
+ However, this only works for types whose names are also valid as
+ function names. For example, double precision
+ can't be used this way, but the equivalent float8
+ can. Also, the names interval, time, and
+ timestamp can only be used in this fashion if they are
+ double-quoted, because of parser conflicts. Therefore, the use of
+ the function-like cast syntax leads to inconsistencies and should
+ probably be avoided in new applications.
+
+
+
+
+
Scalar Subqueries
+
+ A scalar subquery is an ordinary
+ SELECT in parentheses that returns exactly one
+ row with one column. The SELECT query is executed
+ and the single returned value is used in the surrounding value expression.
+ It is an error to use a query that
+ returns more than one row or more than one column as a scalar subquery.
+ (But if, during a particular execution, the subquery returns no rows,
+ there is no error; the scalar result is taken to be NULL.)
+ The subquery can refer to variables from the surrounding query,
+ which will act as constants during any one evaluation of the subquery.
+ See also .
+
+
+ For example, the following finds the largest city population in each
+ state:
+SELECT name, (SELECT max(pop) FROM cities WHERE cities.state = states.name)
+FROM states;
+
+
+
+
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